In physics and fluid mechanics, a boundary layer is the layer of fluid in the immediate vicinity of a surface of interest, where the effects of viscosity may be significant. On an aircraft wing, for example, the boundary layer is the part of the flow closest to the surface of the wing, where viscous forces distort the surrounding non-viscous flow.
In high-performance aerospace and aeronautics designs, such as those for gliders and commercial aircraft, significant resources are devoted to controlling the behaviour of the boundary layer to minimize drag. Two effects should to be considered in this effort. First, the shear forces at the surface of the wing create skin friction drag. Second, the boundary layer adds to the effective thickness of the body, via the displacement thickness, thereby increasing the pressure drag. Third, the boundary layer can separate in a region of local deceleration, creating drastic, often detrimental changes in aerodynamic performance. In order to control, or even understand, boundary layer behaviour, it is therefore desirable to measure and/or calculate fluid pressures, skin friction, and other properties of airflow past a surface of interest.
Stanton gauges are commonly used to measure local skin friction coefficients (and thereby also at least help to determine boundary layer state). The advantage that a Stanton gauge has over other similar, known methods, such as Preston tubes, is that the Stanton gauge calibration is much less dependent upon boundary layer state, making the analysis of the data much simpler. However, Stanton gauges require existing surface static pressure taps to obtain the pressure at the Stanton gauge, which is a modified or disturbed pressure. A local static reference pressure—and, preferably, temperature—are also used to calculate the skin friction in a known manner. Though most wind tunnel models have integral static pressure taps, flight test assets—whether a model or an operational aircraft—rarely do.